Processes for the preparation of stable, aqueous polyurethane-polyurea dispersions are already known and are described, for example, in German Patent Specifications No. 1,184,946 and 1,178,586; German Auslegeschrift No. 1,237,306; German Offenlegungsschriften Nos. 1,495,745; 1,595,602; 1,770,068; 2,019,324 and 2,314,512 and U.S. Pat. Nos. 3,388,087; 3,461,103; 3,479,310; 3,756,992 and 3,686,108 and by D. Dieterich et al, in Angew. Chem. 82, 53 (1970). The dispersions described are based on the principle of incorporating hydrophilic centers into a macromolecular chain of a polyurethane (polyurea) molecule. In the known dispersions, these hydrophilic centers or so called internal emulsifiers are polyether segments containing ethylene oxide groups or ionic groups. The hydrophilic centers may either be built into the prepolymer in the form of special diols or contributed by modified amines used for chain-lengthening the prepolymers which have each at least two isocyanate end functions.
Various processes may be used for preparing the known dispersions and have been described, for example, in D. Dieterich and H. Reiff, Angew, makromol. Chemie 26, 85 (1972). Generally, the solution of a polyurethane in an organic solvent either is converted into an aqueous dispersion or a prepolymer stage in the liquid form, with or without solvent, is dispersed in water. For example, a liquid prepolymer ionomer containing isocyanate groups may be introduced into water with vigorous stirring, in which case an emulsion of the prepolymer is first formed, and this then continues to react by a process of chain lengthening, for example with a diamine or polyamine dissolved in water, to form a high molecular weight polyurethane urea. Since the isocyanate prepolymer is hydrophilic and, therefore, highly compatible with water, this reaction is invariably accompanied to a greater or lesser extent by an unwanted reaction of isocyanate groups with water, which leads to an uncontrolled increase in the size of the molecule. This side reaction increases with increasing dispersion temperature and may even cause gelling of the reaction mixture.
The present invention provides a new process for the preparation of aqueous dispersions or solutions of isocyanate polyaddition products, by which such side reactions are virtually eliminated. According to the invention, urethane prepolymers which contain amino or semicarbazide end groups and which are hydrophilic, i.e. soluble or dispersible in water, are chain lengthened in an aqueous phase with a hydrophobic chain lengthening agent, preferably a hydrophobic diisocyanate. Owing to the hydrophobic character of the chain lengthening agent, virtually no side reaction takes place with the water which constitutes the continuous phase.